Masteries

Offense:
9

Defense:
0

Utility:
21

I will cover the build in stages, linking to leaguecraft snapshots of how your Gangplank should look at various stages in the game. Runes will be displayed in the snapshots as well, but for thoroughness I will list them first. I will also add some "Commentary" to illustrate the logic behind the items and how they compare to the "old way" that many of you have become fond of. For now, I believe the same basic strategy applies: Parrrley until it is safe to melee.

The reason for Magic Resist over something like Dodge for Seals is because I usually kite the enemy DPS with superior mobility, so Dodge isn't necessary. The real threat in team battles is enemy AOE. Now that Parrrley damage is physical, the only way you will do any respectable damage with it is to get armor penetration runes. This is because enemy armor naturally increases as the enemy increases in level, while magic resist remains constant.

Items:
For items, the stats that Gangplank needs to fulfill his support damage role are: Armor Penetration, Cooldown Reduction, Chance to crit, Damage and Health (among other forms of survivability). The described item build attempts to maximize all of these things.

Stage 1 (starting items):
x2http://leaguecraft.com/builder/Gangplank/2d8a9b678fdb368e0a9818c417861f42
Goals: Farm with Smite and Parrrley. Try and stay in your lane long enough to get to Stage 2 items.
Commentary: Starting mana regen remains about the same. Health regen suffers, but you make up for it with an extra health potion. Early game crit chance is dramatically less than before, but with the right lane partner, you should still be able to grab some early kills, especially now that you have so much armor penetration to start. Melee is quite viable in a lot of level 1-3 situations. Look for when your lane opponents overextend themselves. Most players won't expect Melee + Parrrley + Ghost to drop them before they return to the safety of their tower. Health potions will be your primary source of HP regen for the remainder of the laning phase. Buy more when needed.

Stage 2:

http://leaguecraft.com/builder/Gangplank/fdcd88af6a93a7f3a97f4984a302e4ec
Goals: Use Smite and grab Golem buff. Begin using Parrrley non-stop to farm and harass your lane opponents. Establish lane control. Be mindful of other lanes that could use a Cannon Barrage to gank or push down a tower, or to thwart enemy ganks in other lanes.
Commentary: With the improved damage scaling on Parrrley, I have decided it is most prudent to rush an Infinity Edge. Missing out on chance to crit is okay so long as you can make Parrrley non-crit for a fair amount of damage early on. You can upgrade the Meki Pendant to a Chalice of Harmony, but I find I don't need it that much early. Later in the game, if I have 500 gold laying around, I will upgrade the to a Chalice for a little extra magic resist. Finishing an Infinity Edge fast (before 20 minutes if you can) is the way to go. The faster you get it, the more likely you are to snowball on the enemy.

Stage 3:

http://leaguecraft.com/builder/Gangplank/8ad51d16670ccd0e24f333064255cb03
Goals: Team battles and pushes will start taking place, make sure to Parrrley the enemy DPS and use Raise Morale when appropriate. Use Cannon Barrage once the enemy is committed to the fight. Continue to grab golem buff if possible.
Commentary: Finish that Infinity Edge asap to take advantage of the buffed damage scaling. Even though you are no longer doing magic damage with parrrley, you still cannot neglect the awesome potential of cannon barrage, and hence should get Sorcerer's Shoes in anticipation of upcoming team battles. Other boots provide marginal benefit to Gangplank at this point in the game. It works well to grab it around the time that your Cannon Barrage hits level 2. Build up your chance to crit by grabbing another Cloak of Agility.

http://leaguecraft.com/builder/Gangplank/a3eee82415debd80e7821b044cb43050
Goals: The enemy's natural armor should be increasing now beyond your current penetration capabilities. Build a Last Whisper to keep up with their armor growth. Get your crit chance up more by grabbing another Cloak of Agility or Zeal. The Cloak of Agility gives you more chance to crit, the Zeal gives you some movespeed and a number of Stage 5 build options. Try to end the game now with your team, if possible. Your tremendous mid-game advantage will start wearing off. A well-executed Cannon Barrage could help win your team the game here. Winning one big team battle usually results in the enemy losing one or more inhibitors.
Commentary: Basically this part of the game is about grabbing more damage, armor pen, and chance to crit. This is the core build. Most games should end here. If the game doesn't end you will need to make some end-game decisions. Be prepared for Stage 5. HP and survivability will be crucial in this next stage.

http://leaguecraft.com/builder/Gangplank/fb838d8cbc335ea74eeabc6fb4320f7e
Goals: Games can be decided upon how well you can use Cannon Barrage; Don't let your team down. Your mid-game advantage should be wearing off by now, and you will need a serious HP boost to keep doing damage. Gangplank will need HP and a targeted slow at this point because of his new dependence on doing melee damage in addition to parrrley. Sell the Chalice and grab a Phage. At this point you have several options, depending on how you built in Stage 4:

Grog Soaked Blade
As of this current patch, Parrrley no longer applies the Grog Soaked Blade debuff. Note that healing reduction will not stack with any other healing reduction debuffs (e.g. Ignite, Executioner's Calling).

Prime targets upon which to apply this debuff are Alistar, Dr. Mundo and Nasus.

Parrrley
Helps you farm. In team fights, use it on the enemy carry. Try to stay back and avoid enemy fire in the early parts of the battle. Parrrley damage is now physical. See my Parrrley Mechanics Guide to give yourself a better idea of how the damage is calculated.http://leagueoflegends.com/board/showthread.php?t=34529

Remove Scurvy
For escaping, Ghost and this ability will get you out of some ganks. Good players, however, will space out their stuns in a gank so that Remove Scurvy won't get you very far. In team fights, use it to keep yourself out of heavy AOE fire.

Raise Morale
In a lane, it can be used to deny minions, but I prefer to use the mana to farm or harass with Parrrley. Forcing a lane opponent to heal denies them much more gold/xp than killing a few allied minions. Use before a team fight if possible. Always use it to push towers.

Cannon Barrage
AP scales poorly on this ability. Magic penetration, however, will give it some extra mileage. In team fights, wait until the enemy has committed to the fight before laying it down on the battlefield.

* OR *
+ /or/ /or/ /or/

Commentary: In general, you want: Core build + Frozen Mallet/Trinity Force + Phantom Dancer/Other Defense Items. Trinity Force can actually give you a viable damage boost. In my opinion, since the nerf to Sheen, an early Sheen is no longer viable, but getting a Trinity Force in a late game situation could greatly improve your odds of winning. It's obviously all quite situational at that point. Stage 4 will determine how you build in Stage 5.
Summoner abilities:
Smite
I used to get teleport, but Infinity Edge and Trinity Force are expensive, and in high level games with roaming 5-man gank squads you won't have much opportunity to farm. This allows you to maximize the amount of gold you get out of the initial laning phase. With one point in both improved smite and presence of the master, this should have 60 seconds of cooldown, which means you get a free minion kill every other creep wave (with bonus gold). Try to synchronize the cooldown with the appearance of siege minions in the waves, since siege minions will give you more gold bounty. Some lanes will be tough matchups for you, and you won't be able to parrrley every minion in each wave. With smite, I can get my core item build several levels earlier and that's why I take it.

Ghost
I most often use this to escape ganks, or catch up to an enemy to deal a killing blow with Parrrley. In team fights, I use it to kite the enemy if they try to focus me. They will quickly give up since otherwise the rest of your team will destroy them. Cannon Barrage should have them slowed, so it shouldn't be too hard to kite them.

Other option:
Exhaust
Exhaust a viable option in a team battle to cripple the enemy physical DPS. Spending one point in improving it can increase the damage you do with Parrrley by reducing the target's armor, but usually, it will just be for one shot. Smite and Ghost are my prime picks most of the time.